#include<bits/stdc++.h>
using namespace std;
int main(){
    string s;
    cin>>s;
    int n=s.size();
    int ans=n;
    int i=0,j=n-1;
    while(i<j){
        while(i<n&&s[i]!='A'){
            i++;
        }
        while(j>=0&&s[j]!='B'){
            j--;
        }
        if(i<j&&s[i]=='A'&&s[j]=='B'){
            ans-=2;
        }
        i++;
        j--;
    }
    cout<<ans;
    return 0;
}